The present invention relates to the detection of a short circuit between a first and a second electrical line and, in particular, to the detection of a short circuit between measuring lines of a measuring bridge.
Many physical parameters are generally measured by means of a transducer configured in a measuring bridge 10 as depicted in FIG. 1. The bridge 10 comprises a first arm with a series connection of a first reference impedance Z.sub.R1 and a second reference impedance Z.sub.R2 between a node 1 and a node 4 of the bridge 10. A second arm connected between the nodes 1 and 4 comprises a third reference impedance Z.sub.R3 in series with a measuring impedance Z.sub.M. A measuring voltage is taken as a measuring signal S.sub.M between a first measuring node 3 (between Z.sub.R3 and Z.sub.M) on a line 13 and a second measuring node 2 (between Z.sub.R1 and Z.sub.R2) on a line 12. A reference voltage V.sub.D is placed vertical across the bridge 10 between the nodes 1 and 4. A difference in the impedance Z.sub.M changes the signal S.sub.M which is normally amplified by an amplifier 20, e.g. a difference amplifier, and might be analog/digital converted by an A/D converter 30 or otherwise processed as required.
In many applications, it is important to know whether the bridge 10 is working properly or not. Faults in the bridge 10 may result from open wires or from short circuits. Commonly used techniques apply pull-up resistors or current sources, e.g. current sources 40 and/or 50 in FIG. 1, from plus (+V.sub.D) or minus (-V.sub.D) voltage supply to the nodes 2 and/or 3. This allows to detect short circuits between the nodes 2 or 3 and the voltage supply or a ground level. Open circuits can be detected accordingly as maximum or minimum voltages of the measuring signal S.sub.M.
However, none of the fault detection techniques as known in the art allows a direct detection of a short circuit occurring between the nodes 2 and 3, e.g. when nodes 2 and 3 are tight together. There is no difference in the measuring signal S.sub.M between the short circuit between the nodes 2 and 3 and a zero value signal from the bridge 10. The only possibility to detect such a fault is by means of a plausibility check, i.e. by observing the variability of the measuring signal S.sub.M in combination with the expected characteristics of the signal (e.g. dynamics, repetition, etc.). However, it is clear that in particular in cases of slowly changing signals, such as pressure, force etc., which can remain for a long time at values near to zero, the plausibility check is not sufficient. Furthermore, a plausibility check has to be ruled out in case that the characteristics of the expected measuring signal S.sub.M is not known, or generally speaking, when no knowledge about the signal to be measured is available.